1. Field of the Invention
The present invention relates to a nickel-metal hydride secondary cell and more particularly, to one having an improved hydrogen absorbing negative electrode.
2. Description of the Related Art
As recent electronic technology has been advanced, ensuring lower energy consumption and improved mounting techniques, most electronic components which were unquestionably bulky become cordless and portable. Consequently, various secondary cells which serve as built-in power sources in electronic components have also been required to hold a higher amount of electrical energy. Among them, an alkali secondary cell which has a negative electrode fabricated by forming a powdered hydrogen absorbing alloy on a conductive core which serves as a collector has been developed and widely accepted. As compared with the cadmium negative electrode in a known alkali secondary cell, such a hydrogen absorbing alloy type negative electrode provides a higher rate of energy density in weight or volume, thus ensuring higher energy storage in a cell. In addition, the characteristics of the cell can be improved and will be less assaultable on the environment.
It is known that the hydrogen absorbing alloy negative electrode of the nickel-metal hydride secondary cell is constructed of a non-sinter type by filling a 3-dimensional conductive core, which is made of foamed metal, sintered fiber, or the like and serves as a collector, covered with a powder of hydrogen absorbing alloy and a binding medium. However, the hydrogen absorbing alloy negative electrode which utilizes such a conductive core has less mechanical strength and may be destroyed in the press forming step during production. It is also disadvantageous that the conductive core is so expensive that the production cost of a nickel-metal hydride secondary cell having such a negative electrode increases inordinately.
For the purpose of overcoming the aforementioned disadvantage, a non-sinter type hydrogen absorbing alloy negative electrode is disclosed in U.S. Pat. No. 4,849,312, in which the collector is made of a portion of punched metal having a 2-dimensional structure, which is less expensive to produce than the above 3-dimensional conductive core. Such a hydrogen absorbing alloy negative electrode is composed of a powder produced by mixing, at high speed, a hydrogen absorbing alloy with polyacrylate, polytetrafluoroethylene, and carboxymethylcellulose preparing a paste therefrom, and applying the paste to the portion of punched metal which is in turn dried and pressed into shape.
It is, however, difficult to increase the utilization of the non-sinter type hydrogen absorbing alloy negative electrode as compared with the negative electrode having a 3-dimensional conductive core as a collector, as the former does not allow the nickel-metal hydride secondary cell to provide a significantly higher capacity. Also, during the pressing step in the production of the negative electrode, the collector made of punched metal may be fractured or the dried paste may be partially separated from the punched metal.